Car Motors and Radio Waves

To play music from my iPod on my car stereo, I use a small radio transmitter: I plug a the device into my mp3 player, set it to a certain radio frequency, turn the car radio to the same frequency, and voila.

What I realized this week is that the motor turning affects the signal: the stereo carries a high whine when the engine turns, it grows in volume as the motor turns faster. I can watch the tachometer needle (which gauges RPM) rise and drop as I accelerate and the transmission shifts gears; I hear the whine crescendo and decrescendo accordingly.

I wonder what causes this.

Yes, the radio transmitter plugs into the cigarette lighter, so it draws power from the car battery, but its primary signal (the music) does not crescendo and decrescendo in time with the speed of the motor.